Nuclear reactor power monitor



Jain. 12, 1965 s. UNTERMYER n NUCLEAR REACTOR POWER MONITOR 2Sheets-Sheet. 1

Original Filed Feb. 13, 1959 glllmlallll A INVENTOR. SAMUEL UNTERMY R EV v A I ran/vs)- Hll llllllllll Jan. 12, 1965 s. UNTERMYER u NUCLEARREACTOR POWER MONITOR Original Filed Feb. 13, 1959 v 2 Sheets-Sheet 2 nm U L w M J BY i I {I United States Patent Office 3,165,446 PatentedJan. 12,1965

3,165,446 NUCLEAR REACTOR POWER MONITOR Samuel Untermyer II, Atherton,Calif 'assignor to Gem eral Electric Company, a corporation of New YorkContinuation of application Ser. No. 793,117, Feb. 13, 1959. Thisapplication Dec. 26, 1962, Ser. No. 247,404 11 Qlaims. (Cl. 176-26) Thisinvention relates to the operation of nuclear reactors, and inparticular it relates to an improved method and apparatus for monitoringthe rate of energy liberation or power level at various locationsthroughout the length and across the transverse cross-section of anuclear reactor core.

The release of large amounts of energy through nuclear fission reactionsis now quite well known. In general, a fissionable atoms such as U U orPu absorbs a neutron in its nucleus and undergoes a nucleardisintegration. This produces on the average, two fission products oflower atomic weight and great kinetic energy, and several fissionneutrons also of high energy. For example, the fission of U produces alight fission product and a heavy fission product with mass numbersranging between 80 and 110 and between 125 and 155 respectively, and anaverage of 2.5 neutrons. The energy release approaches 200 mev. (millionelectron volts) per fission.

The kinetic energy of the fission products is quickly dissipated as heatin the nuclear fuel. If after this heat generation there is at least onenet neutron remaining which induces a subsequent fission, the fissionreaction becomes self-sustaining and the heat generation is continuous.The heat is removed by passing a coolant through heat exchangerelationship with the fuel. The reaction may be continued as long assufficient fissionable material exists in the fuel to override theeifects of the fission products and other neutron absorbers which alsomay be present.

In order to maintain such fission reactions at a rate sufiicient togenerate useful quantities of thermal energy, nuclear reactors arepresently being designed, constructed, and operated in which thefissionable material or nuclear fuel is contained in fuel elements whichmay have various shapes, such as plates, tubes, or rods. These fuelelements are usually provided on their external surfaces with acorrosion-resistant non-reactive cladding which contains no fissionableor fertile material. The fuel elements are grouped together at fixeddistances from each other in a coolant flow channel or region as a fuelassembly, and Sllfi'lClEllll fuel assemblies are combined to form thenuclear reactor core capable of the self-sustained fission reactionreferred to above. The core is usually enclosed within a reactor vessel.

The control of nuclear reactors is ordinarily atfected by means of amovable control element placed in the core and containing a nuclearreaction poison, that is, a material containing a significant number ofatoms which have relatively large non-fission neutron capture orabsorption cross-sections. The control problem is unusually difficult inpower reactors of high thermal rating and which have large cores. Insuch cores, relatively large local changes in reactivity and power canoccur in a region surrounding the particular control element which ismoved without 7 significant affect upon the total reactor power. Controlelements disposed uniformly through such a core are required to controlsuch local regions more or less individually and thus maintain suitablecontrol over the reactor;

ual regions of the core are operating at given time so that many controlelements may be moved simultaneously to avoid local hot spots ofexcessively high power.

Previously. this power level determination has been affected in severaldistinct ways. In some reactors which are provided with nuclear fuelcontained in individual coolant flow channels, the temperature rise ofthe coolant as it passes through a given channel is measured. Thistemperature increase, together with the coolant flow rate, permits adetermination of the power level at which the fuel in that particularchannel operates. A second systern includes the placement in the reactorcore of thermal neutron detectors, such as a thermocouple coated with afissionable isotope or an ion chamber lined with such an isotope, topermit a determination of the thermal neutron fiux. This value, takenwith the known concentration of fissionable isotopes in the fuel,permits an estimation of the power level in a given part of the reactor.A third power monitoring system involves the placement in the core offoils or wires made of materials, such as'copper or indium for example,which are activated in a manner and to an extent dependent upon theneutron flux or power level at the point of placement. A fourth powermonitoring system which is adapable to boiling reactors involvesremoving a sample of the mixed phase coolant from the core and passingit through a calorimeter to determine the heat content of the eflluentcoolant. There are, of course, other systems which may be and have beenused.

Unfortunately, all of the power monitoring systems referredto abovesuffer from one or more serious disadvantages. They all requirepenetration of the reactor pressure vessel with electrical or otherinstrumentation leads with the possible exception of the third systemwhich employs foils or wires within the core. This system, however,requires that the reactor be shut down and the foils or wires removed inorder to determine the power level information. Further, those systemsrelying upon the irradiation of detectors containing fissionablematerial have to be recalibrated frequently since the fissionablematerial is consumed during the period of use. The measurement ofcoolant temperature rises is completely inoperable in a reactor wherepart of the coolant is vaporized in the coolant channel. Otherdisadvantages of these systems are well known to those who haveattempted to obtain accurate results in applying them to the powermonitoring of a nuclear reactor.

It is accordingly an object of this invention to overcome thesedisadvantages and provide an improved method and apparatus fordetermining the relative power and power distribution throughout anuclear reactor core without the necessity of pressure vesselpenetrations, the placement of extraneous or foreign materials in'thereactor, or interfering with continuity of reactor operation.

It is a further object to utilize the penetrating radiation emitted froma nuclear reactor core to generate a signal which is proportional to theactual local power level in a given region of the core, the signal beingsuch that a relatively large error in the determination of thesignaldoes not interfere with an accurate determination of the localpower level.

An additional object is to provide for the control of large nuclearpower reactor cores to maintain uniform or other predetermined powerdistributions therein.

Another object of this invention is to provide a nuclear reactor powermonitoring system in which all sensitive equipment and detectors aredisposed remotely from the core, such as outside the reactor vessel, sothat they may be maintained and serviced during reactor operation.

A further object of this invention is to provide a power monitoringsystem in which a single coolant flow channel or a section of. a reactorcontaining a plurality of coolant flow channels is monitored by means ofa simple mechan- 39 ical arrangement which may be located entirelyoutside of the reactor vessel, which does not require vesselpenetrations, and which does not interfere with continuous reactoroperation.

Other objects and advantages of this invention will become apparent tothose skilled in the art as the description and illustration thereofproceed.

Briefly, the present invention comprises a method for operating anuclear reactor, and a nuclear reactor power level monitoring method andapparatus including the steps of and means for collimating thepenetrating radiation emitted from a nuclear chain reacting assembly orcore, and which passes through any pressure vessel or container and anycoolant or moderator which may be present along the radiation path, anddetecting the collimated radiation by means of a radiation detectorlocated remote from the reactor core to measure the radiation in tensityto provide a, signal or other information bearing a predeterminedrelation to the rate of energy liberation or power level in a particularregion of the core from which the detected radiation is emitted. Thecore region monitored may be a single selected nuclear fuel-containingcoolant flow channel, or a group of channels adjacent one another.Significant information can be obtained by viewing any radiating surfaceof a reactor core. The radial power distribution is most preciselyobtained by detecting collimated penetrating radiation emitted axiallyfrom an end of each coolant iiow channel in the core to obtain therelative power level of each channel. The axial power distribution canbe determined by detecting the collimated penetrating radiation emittedlaterally from the lateral surface of the core at a sufficient number ofpoints on an imaginary cylinder coaxially surrounding the reactor core.

The radiation attenuation or absorption varies with changes in thedensity and therefore the temperature of the coolant or the moderatorthrough which it passes. The coolant temperature in turn varies with thepower level of the fuel in the coolant flow channel or channels fromwhich it issues whereby the collimated radiation has an intensity whichis a direct function of the power level. The variation in attenuationwith temperature occurs in all liquids; it is pronounced in hydrogeneousliquids, such as the hydrocarbons, water, etc., and it is extreme in thecase of a boiling reactor where vapor voids are present. Either singleor multiple and either stationary or movable detectors may be used tosurvey the penetrating radiation according to this invention. Thecollimation may be provided either immediately adjacent the reactorcore, or immediately adjacent the detector, or both. The penetratingradiation contemplated includes fast neutron radiation, gamma rayradiation, or both.

The present invention will be more readily understood by reference tothe accompanying drawings in which:

FIGURE 1 is a schematic elevation view of a nuclear reactor systemincluding the simplest embodiment of the present invention;

FIGURE 2 is a transverse cross-section view of the reactor vessel andcore shown in FIGURE 1;

FIGURES 3, 4, and 5 are schematic illustrations of the theory underlyingthe operation of the present invention;

FIGURE 6 is a schematic illustration of the penetrating radiationemitted from a reactor core and detected by a collimated detector;

FIGURE 7 is a cross-section view of another embodiment of the presentinvention applied to a large nuclear power reactor;

FIGURE 8 is a detail view of the cross-section of the internalcollimating means used inside the reactor pressure vessel illustrated inFIGURE 7;

FIGURE 9 is a detail longitudinal view of the crosssection of anexternal collimating means suitable for use in this invention; and

FIGURE 10 is a cross-section view of the collimator shown in FIGURE 9.

In FIGURE 1, nuclear reactor pressure vessel 10 is provided withremovable head 12 connected by flanges l4 and 16. Reactor core 18,consisting of a plurality of individual nuclear fuel-containingassemblies or coolant flow channels, is supported within vessel 19 andmay be provided with a chimney section 20 to enhance circulation of thecoolant through the core. A representative fuel assembly 22 is indicatedby broken lines. Control element 24, containing a nuclear reactionpoison, extends upwardly into core 18 in order to control the nuclearfission reaction and is positioned by means of a control element drivemechanism 26. Core 18 is surrounded by a thermal shield 23 spaced apartfrom the inner wall of vessel It) opposite core 18 to inhibit gamma rayheatingof the pre sure vessel.

Referring now briefly to FIGURE 2, a transverse section view of thenuclear reactor shown in FIGURE 1 is indicated. Pressure vessel It)surrounds thermal shield 23 which in turn surrounds the nuclear reactorcore 18 made up of a more or less circular group of parallel nuclearfuel-containing assemblies 22 of square cross-section and through whichthe moderating coolant passes.

Returning again to FIGURE 1, coolant inlet 38 and coolant outlet 32 areprovided by means of which a liquid moderating coolant is interduced andremoved from the reactor vessel. The reactor illustrated in FIGURE 1 isof the boiling coolant-moderator type. Liquid coolant is introducedthrough line 3i at a rate sufiicient to maintain the body of liquid inthe reactor vessel having an upper level 34 as indicated. The liquidflows downwardly through the more or less annular spaces on each side ofthe thermal shield 28, upwardly through the various fuel assemblies 22,and into the upper part of the liquid body where the vaporized portionis separated through liquid level 34. The liquid recirculates by naturalconvection in the direction indicated by the arrows. The vapordischarges into the vapor plenum 36, passes around deflector 38, anddischarges through outlet line 32. A suitable moderating coolant isnatural water.

Disposed immediately above the reactor pressure vessel is collimatorcontaining detector element 42 which in turn is connected to instrumentmeans 44 provided with indicator 46. Collimator 46 consists of anelongated element having heavy walls and provided with a central openingor bore 48 of relatively small diameter which is axially aligned with oraimed at channel 22 in core 18 along radiation path 50. In fuel assembly22 is an effective source 52 of penetrating radiation. This radiation isemitted in the direct line along path into collimator bore 48, as wellas in all other directions, one of which is represented by path 54.Another representative source of penetrating radiation is indicated at56 in another part of core 18 and out of direct path 50. A plurality ofsuch collimators may be employed to monitor various parts of core 18 andan additional such collimator 40' is also shown.

Part of the penetrating radiation fromsource 52 passes upwardly alongdirect path 50 through the effluent coolant from fuel assembly 22 andinto collimator bore 48 where it actuates detector 42. The penetratingradiation emitted from source 52 other than along direct path 50, suchas along paths represented by path 54, does not enter collimator bore 48and does not actuate detector 42. Radiation from other parts of core 18,suchas that from source 56, which may be emitted along a direct path 58toward detector 42 does not enter collimator bore 48 because of themisalignment of path 58 with the bore, and further does not efiectivelyactuate detector 42 because it is attenuated to a significant extent inpassing more or less longitudinally but at a slight angle through thewalls of collimator 40. The collimator walls are made of materialselected to attenuate efliciently all such extraneous radiation.Accordingly, by far the strongest signals generated by detector 42result from penetrating radiation emitted from sources such as 52existing in the fuel assembly 22 directly aligned with bore 42 of thecollimator.

The degree of attenuation of this penetrating radiation as it passesalong path 50 is strongly affected by the density and therefore thetemperature of the coolant and/ or moderator present along the path. Thevpresence of high temperature coolant, and especially the presence ofvoids of vaporized coolant, along this path reduces the attenuation fromthat which would occur if the coolant were colder or were without voids.Accordingly, the signal generated at detector 42 from radiation source52 is an inverse function of the coolant or moderator density and adirect function of the coolant or moderator temperature and the rate ofenergy release or power level in the fuel assembly monitored. Due to theeificient shielding effect of the walls of collimator 4d, extraneousradiation from sources outside of fuel assembly 22 is attenuated.

Detector 42 is connected by suitable means to instrument 44 in which thedetector signals may be amplified, various signals may be discriminatedon the basis of radiation energy, and the signals may be rectified toproduce a direct current or other signal which is proportional to theintensity of the radiation arriving at detector 42. This signal may beused to actuate indicator 46 to give a reading which is a direct measureof the relative power level in fuel assembly 22.

As applied to a boiling reactor, which is moderated and cooled bynatural water, that is, the natural mixture of water isotopes, thepresent invention operates as follows. The present invention measuresthe outlet steam voids from boiling channel 22 by measuring theattenuation of penetrating reactor radiation in passing through thesteam-water mixture leaving channel 22 in the reactor core. Whereaseither gamma or neutron radiation might be used, in a preferredembodiment a fast neutron detector such as a Hornyak button, hereinaftermore fully described, or a U fission counter, is used. These detectorscan be adjusted by means of a suitable discriminator circuit to measureonly fast neutrons above 1 m.e.v., and they can be set to reject othertypes of radiation such as fission gamma rays. This fast neutrondetector is mounted at a convenient location above the top of thereactor pressure vessel. Fast neutrons are produced by fission of fuelnear the top of the reactor core in fuel assembly 22. In order to reachdetector 42, these neutrons must pass along path 54 through a sample ofsteamwater mixture discharged from fuel assembly 22. They pass through acollimator 40 which is aligned with or aimed at the fuel assembly flowchannel to be monitored. While neutrons may also reach detector 42 bybeing scattered into the line of the collimator from other parts of thereactor, such neutrons will be attenuated by the additional distancethey must travel through the coolant and they will be reduced in energydue to the scattering so that they will not be counted by the detector.

In FIGURES 3, 4, and are shown highly schematic diagrams illustrating anuclear fuel-containing coolant flow channel to explain the variation inattenuation of the penetrating radiation as a function of the powerlevel at which the fuel in the channel operates. In these figures theflow channel walls are indicated at 80 with the coolant being introducedat the bottom and discharged at the top in the manner indicated. Thenuclear fuel is contained between the walls 80, but is not shown sinceillustration is unnecessary to the explanation. The body of liquidcoolant is indicated as having a level 82 and in FIGURES 4 and 5 thepresence of vapor voids 84 and 86 indicate the coolant is being boiled.In each case a source 88 of penetrating radiation is indicated in thechannel.

FIGURE 3 may be taken to illustrate the situation which exists in anuclear reactor through the flow channels of which the coolant passeswithout boiling. The coolant temperature rises from some inlet value toa high outlet value below the boiling point as it passes through thechannel. The attenuation of penetrating radiation is a direct functionof the number of collisions with atoms 6 of the material or of theopacity of the material through which it passes. For example, fastneutron radiation emitted at source 88 and progressing along path 90 isattenuated to an extent which is a direct function of the density of thecoolant. As the power level rises, the temperature rise of the coolantin the channel increases, the density of the coolant discharging fromthe channel decreases, the attenuation of the radiation decreases, andthus the radiation intensity at the point of detection increases. Thedetector signal is thus a direct function of the power level.

All liquid coolants normally used to remove heat from or moderatenuclear reactors have densities which decrease with increasingtemperature. The present invention, therefore, is applicable to monitorthe power of these liquid cooled or liquid moderated nuclear reactors.Some representative nuclear reactors include liquid materials such asbismuth, lithium, mercury, sodium, potassium, the NaK alloy, thehydrogeneous liquids such as terphenyl, diphenyl, other refractoryhigh-boiling hydrocarbons, light or natural water, heavy water, and thelike.

FIGURE 3 also illustrates the non-boiling condition which exists in aboiling reactor prior to attainment of temperatures sufiicient tovaporize the coolant. For example, at 1000 p.s.i. operating pressure,the boiling point of pure water is about 545 F. At 540 F., for example,no net power is liberated from such a reactor since no steam isgenerated. At this point, however, the attenuation of radiation fromsource 38 in passing upwardly through the non-boiling channel is somefixed value dependent upon the intensity of the source and the distancethrough the hot non-vaporized coolant to liquid level 82.

When the power level is increased slightly, the increased heat liberatedraises the temperature to the saturation V value, vaporizes part of thecoolant, and the condition which then exists is shown in FIGURE 4. Heresteam coolant vapor voids 84 form and rise through channel toward liquidlevel 82 due to their own buoyancy and the net upward movement of theliquid coolant by convection in the channel. Voids 84 along theradiation path 92 present regions of substantially decreased density andtherefore substantially less attenuation of the radiation occurs inthese regions. Compared with the non-boiling condition, as shown inFIGURE 3, the attenuation is lower in FIGURE 4 and accordingly thedetected radiation intensities will be higher.

A further increase in power level has the effect of increasing the sizeof the vapor voids 85 as shown in FIG- URE 5. With the higher powerlevels, the fraction of vapor in the coolant discharged from channel 89increases, still less attenuation of the radiation occurs as it passesfrom source 88 along path M, and the detected radiation intensity isstill higher.

The power monitoring system of this invention thus may be used to detectdifferences in power level in both non-boiling and boiling reactors, andis particularly sensi ,tive to power level changes in the boiling typeof reactor because a greater change in the degree of attenuation iscaused when coolant voids are formed.

EXAMPLE I This invention was tested in the operation of a nuclear powerreactor rated at 25 thermal megawatts and which was moderated and cooledby boiling natural water. The detector element was a I-Iornyak buttonprovided with a mfld steel collimator and was positioned above the headof the reactor pressure vessel substantially as indicated in FIGURE 1.The reactor corewas provided with seven control elements, six suchelements grouped around the central seventh element. The total reactorpower in this example was varied by moving only the central controlelement while the sixouter elements were completely withdrawn from thecore. The monitor of this invention was aligned with that region'of thecore Table I Reactor Power, Thermal Megawatts O Power, Pcr Cent ofRating 20 60 100 Eflluent Coolant:

Vapor Fraction, Volume Percent 9 24 34 Liquid Fraction, Volume Percent91 76 n 66 Counts per Minute 550 12, 000 3b, 000

Counts per Minute per Megawatt It may be seen from these data that thecounts per minute per megawatt vary in the present inventionapproximately logarithmically with the fraction of vapor phase in theeffluent coolant. Thus at high power levels slight changes in powerlevel result in relatively large changes in the number of counts perminute per megawatt. This means that even a substantial inaccuracy inthe determination of the counts per minute results in only slightinaccuracy in the determination of the actual power level.Correlatively, very precise determinations of power level may be madewith only reasonably exact determinations of the counts per minute.

EXAMPLE II The following data show the response of the power monitor ofthis invention to variations in power level in one part of the core asinduced by moving the various individual control elements whilemaintaining a constant total reactor power of 15 thermal megawatts. Thecollimated detector was aligned with a region of the core near one ofthe six outer control elements. \Vith all but this particular controlelement out of the core, the monitor indicated 6500 counts per minute.With all but the central control element out of the core, the monitorindicated 12,000 counts per minute as stated in Table I above. With allcontrol elements removed from the core except the outer element farthestfrom the region being monitored, the monitor indicated 54,000 counts perminute. These data clearly indicate the ability of the power monitor ofthe present invention to detect local power levels in a reactor core.

In FIGURE 6, an illustration is shown including collimator 41, detector42, and a schematic illustration of a laterally foreshortened reactorcore '72 including fuel assembly 74 with which the collimator is alignedalong path 76 and which is surrounded by a plurality of other fuelassemblies '78 which are out of alignment with collimator bore 43. Thepurpose of this illustration is to describe the basis on which largesignal to noise ratios are obtained according to this invention evenwhen monitoring the relative power of a single flow channel of smalllateral dimensions in a large power reactor core consisting of hundredsof such channels. This selectivity which manifests itself as a highsignal to noise ratio is due to the use of collimator 41 in conjunctionwith the detection at detector 42 of the penetrating radiation emittedby reactor core 72.

When collimator 41 is aligned along path 76 with coolant channel 74 inthe center of a large more or less circular reactor core, penetratingradiation is emitted from the entire upper surface of the core towarddetector 42 through the conical volume between lines 31 and 83. Most ofthis radiation is attenuated substantially in passing at a slight anglelongitudinally through the walls of collimator 4i. Thus, only thepenetrating radiation from the vicinity of channel 74 actuates detector42 since only it is aligned with collimator bore 48 along path 76 inwhich no collimator wall attenuation occurs.

When the collimator 41 is aligned along path 76 with a coolant flowchannel '74 located at or near the edge of a large more or less circularcore, penetrating radiation from the entire upper surface of the core isemitted toward detector 42 through a conoidal volume between lines 76and 35, or between lines 76 and 87, depending upon whether the alignmentis with the left hand side or the right hand side of the core,respectively. Again, nearly all of this radiation is attenuatedsignificantly by passage at an acute angle longitudinally through thewalls of collimator 41, and only that radiation emitted along path 76effectively actuates detector 4 2.

Thus, in the present invention the collimator effectively reducesextraneous penetrating radiation emitted from the core surface towardthe detector along paths other than path 76. Detector 42 is actuatedonly by radiation emitted along path 76 in alignment with the collimatorbore.

Because the exposed annular radiation area which is a given averageradial distance from that part of the core with which the collimator isaligned increases as the square of the radius, the total radiation fromthese displaced areas may be relatively high compared to that emitteddirectly along path '76. Accordingly, greater attenuation in thecollimator walls is necessary for that radiation arriving at largerangles of misalignment. Accordingly, in the present invention the wallsof collimator 40 are relatively thick, especially at the end nearestdetector 42 and may have, in order to reduce weight, a decreasing wallthickness with distance from detector 42 toward the radiation source.Accordingly, the generally tapered shape shown in FIGURE 6 may beemployed, or the stepped shape which also tapers toward the radiationsource as shown in FIGURE 1 may be used. For minimum weight, thecollimators used adjacent detector 42 in this invention approximatehyperboloids of revolution.

The extent to which directpenetrating radiation reaches detector 42 bypassage through the walls of collimator 41 is determined by the lengthand diameter of collimator bore 48, the material of construction, andthe distance between detector 42 and the reactor core 72. The sphericalangle through which such direct radiation may pass along line '76 todetector 42 without passing through the collimator wall illustrated inFIGURE 1 is the apex angle of the conical volume between lines 91 and93. Radiation emitted toward detector 42 along paths outside of thisconical volume, such as along paths in the larger conical volume betweenlines 95 and 96, passes through and is accordingly attenuatedsignificantly by the walls of collimator 41. Direct radiation fromsource 98 in channel 74 is not attenuated by the collimator wallswhereas radiation from sources 100 and 102 in the channels adjacentchannel 74 are substantially attentuated by them. Thus in the systemillustrated in FIGURE 6, detector 42 not only rejects radiation emittedfrom large areas of the reactor core which are displaced radially fromalignment with the collimator bore, but also rejects radiation emittedfrom channels immediately adjacent the channel with which the collimatorbore is aligned.

According to this invention, the extent'of collimation is predeterminedon the basis of the desired selectivity of monitoring, the size of thecore, and the distance from the core to the detector. For example, in apower reactor containing 500 individual nuclear fuel-containing coolantflow channels about 4.5 inches on a side, it is entirely possible toView and monitor any given individual channel in the entire core bymeans herein described. On the other hand, if it is desired to monitorlarger regions, such as square groups of 4, 9, 16 or 25 channels forexample, a lower degree of collimation will permit such an enlargementof the field viewed by the detector according to this invention.Additional detail in this regard is given below in connection with thedescription of FIGURES 7-10.

In FIGURE 7, another embodiment of the present invention is shown asapplied to the power monitoring of a relatively large power reactor inwhich axial and radial power distributions are monitored. Reactor vesselis provided with vessel head 132 connected by means of flanges 134 and136. Coolant inlets 138 and 140 are 9 provided opening into the bottomof vessel 130 and coolant outlets 142 and 144 are provided at the upperportion of the vessel just below flange 136. In the particular reactorillustrated, water is introduced as the moderator-coolant at the bottomof the vessel, the water is partially vaporized in passing upwarlythrough the reactor and a mixture of steam and saturated water isdischarged from the coolant outlets. A downwardly tapering turning vane146 is provided to direct the coolant mixture radially outward throughthe outlets 142 and 144.

The nuclear chain reacting assembly or core 148, made up of a pluralityof fuel-containing assemblies through which the coolant is channeled, issupported by core support means 150 in the lower part of vessel 130. Aplurality of control elements 156 containing a nuclear reaction poisonextend upwardly from control element drive mechanisms 158 to control thenuclear reaction and the power level in the various regions of the core.This reactor illustrated is representative of large reactors havingcores of about 10 feet high and 10 feet in diameter, or larger, andcontaining as many as 40 critical masses of fissionable material. Insuch reactors, substantial local power changes can occur which do notaffect extensively the total power level of the entire core.Accordingly, as shown in FIGURE 7, means are provided for monitoring thepower level of a plurality of regions throughout the entire transversesurface of the core to determine the radial power distribution. Disposedmore or less uniformly throughout this surface above the core is aplurality of internal collimating elements 160. The structure of oneform of these elements is detailed in FIG- URE 8 subsequently described,but in' general each consists of a lower mixture section which is openat both ends and aligned axially with an upper collimating section whichis preferably closed at both ends. The axis of each internal collimatingelement is aligned with the particular region in the core to bemonitored and with an external collimator 162 containing detector 164 asindicated previously in connection with FIGURE 1. A plurality ofstationary collimated detectors may be provided, one for each of theinternal collimating elements 160 used in the pressure vessel. In FIGURE7 a single collimated detector is movably supported by reciprocable arm163 which is in turn slidably mounted on support ring 165. The detectormay thus be moved into successive alignment with each element 160.

An additional collimator 166 having detector 158 may be employed aroundthe periphery of core 148 in order to monitor the power level of aplurality of regions throughout the lateral surface of the core todetermine the axial power distribution. A plurality of stationarycollimated detectors may be distributed throughout the surface of animaginary cylinder coaxially surrounding core 148. In FIGURE 7 thecollimated detector is movably supported by means of reciprocable arm167 which in turn is slidably mounted on support ring 169. Thecollimated detector may thus be moved to any point on the imaginarycylinder in order to determine the axial power distribution of the core.7

In operation, part of the mixture of vapor and liquid coolantdischarging from the entire upper surface of core 148 enters the lowermixture section of collimating elements 160, and passes upwardly to theupper open end where it is discharged into the main coolant stream. Thelower mixture section of tube 160 thus contains a representative sampleof the vapor-liquid coolant mixture discharging from a particular regionor fuel assembly of the reactor core. External collimator 162 is alignedwith one of these internal collimator elements 160, and the penetratingradiation from a source 170, for example, passes upwardly through partof the core, the lower mixture section and the upper collimating sectionalong path 172 through external collimator 162 to detector 164. Theradiation is attenuated to an extent which is an inthe core, a sample ofwhich passes through the mixture section of element 160. Considerableaddedattenuation of misaligned radiation, that is radiation arriving atdetector 164 along paths other than path 172, is obtained by virtue ofthe presence of internal collimating elements 160. It is not necessarythat internal collimating elements be provided with the mixture sectionhowever, and one preferred embodiment of this'invention utilizes aninternal collimator consisting of a tube closed at both ends andextending from adjacent the core to the vessel wall and aligned with thebore of an external collimator.

FIGURE 8 is a detailed longitudinal cross-section view of the internalcollimator tube 160 shown in FIGURE 7. It extends from a point justabove the coolant outlet end 174 of the reactor core through turningvane 146 and to a point adjacent the inner surface of reactor vesselhead 132. The inlet end 176 of tube 160 is open while theother end 178is preferably closed. Intermediate these ends is a transverse divider180 and immediately adjacent the divider one or more openings 182 areprovided in the tube wall. That portion of tube 160 between divider 180and end 17 8 is an empty collimating section 184 While that portionbetween the divider and inlet 176 is a mixture section 186. The sampleof partially vaporized coolant passes upwardly through mixture section186 and discharges through openings 182. The penetrating radiationoriginating in the core at a point 190 passes upwardly along path 192through mixture section 186 where it'is attenuated to an extentdependent upon the degree of vaporization of the coolant and the powerlevel in the particular channel. The attenuated radiation passes throughcollimating section 184 and through vessel head 132 to the superjacentdetecting means, not shown but indicated in FIGURE 7.

Referring now to FIGURES 9 and 10, a detailed longitudinal andtransverse cross-section view, respectively, are shown illustrating thedetail of one form of construction of the detector collimator 162referred to in FIG- URE 7. This collimator is constructed with acenterbore 200 and walls 202 made up of aplurality of concentric mild steelpipe sections each of increasing length to approximate the hyperboloidof revolution previously described. In this particular embodiment ofthis invention, six concentric tubular sections having the following dimensions were used.

Table II Length Nominal (inches) Diameter Schedule (inches) Disposedwithin bore 200 is a bundle of collimating tubes 204, in this case threenominal 0.5 schedule 40 pipes, extending substantially the full lengthof bore 200. Considerable increase in selectivity of the collimatorresults from the addition of this collimating tube bundle. In the upperor heavy end of collimator 200 is provided a position fora radiationdetector 206.

EXAMPLE HI A. boiling water power reactor of the type described inconnection with FIGURE 7 rated at 656 thermal megawatts had 488 fuelchannels 12 feet long and 4.5 inches on a side. Eighteen internalcollimating tubes of the type described in connection with FIGURE 8 andan external detector collimator of the type described in FIGURES 9 and10 were used. The detector was a Hornyak button areas re spaced 360inches from the top of the core. It was determined that signal to noiseratios about 350 were obtained with the device when aligned with theedge of the core and ratios of about 900 were obtained with this devicealigned with the center of the core. It was further determined that thedetector was effectively actuated only by penetrating radiation emittedupwardly from an area only 3 inches in diameter on the top of the core.Effectively then, the embodiment of this invention as described inFIGURES 7 through successfully monitored the power level of a single 4.5inch by 4.5 inch fuel channel in a boiling water reactor core containing487 other fuel channels located 30 feet away inside a 5.5 inch thicksteel reactor vessel.

In the practice of the present invention, penetrating radiation has beencollimated and utilized to monitor the power of small local regions of areactor core without seals or penetrations extending through the reactorvessel and by means of instruments of moderate accuracy located wellaway from the core and outside the vessel.

The penetrating radiation contemplated includes fast neutrons such asabout 1 mev. and gamma ray radiation and mixtures of fast neutron andgamma ray radiation.

The detector elements suitable for use in the practice of this inventiondepends of course upon the nature of the penetrating radiation. For fastneutrons, the conventional fission counter or scintillation counters maybe employed. One particular detector element which has been found verysuitable for fast neutrons is the so-called Hornyak button described byAlfred Hornyak in Review of Scientific Instruments, vol. 23, (1952) atpage 264. The button consists of an optically clear hydrogeneous plasticmaterial to which is added about 10 percent by weight of uniformlydispersed finely divided zinc sulfide. Suitable button may be made bymixing 1.5 grams of 8-25 microns size particles of zinc sulfide and 10grams of polymethyl methacrylate (Lucite) molding powder and molding at2000 psi. and 120 C. to produce a cylindrical button about 1 inch indiameter and about 0.25 inch thick. Visible light is emitted by thismaterial under fast neutron irradiation. The scintillation is detectedby means of a photo multiplier tube, a 5819 is suitable, placed adjacentthe material. The usual electronic instrumentation employed with photomultiplier tubes in radiation detection and counting may be employed.This includes the conventional amplifiers, discriminators, rectifiers,detectors, indicators, recorders, and the like.

With gamma ray radiation, scintillation counters may be used inconjunction with a sodium iodide crystal, or the more conventional ionchambers and Geiger tubes may be employed.

In either case, it is conventional to employ discriminator circuits sothat signals generated by radiation having energies below somepredetermined limit are not effective in actuating the recording andindicating devices. For example, neutrons below about 1 mev. includingthe extraneous radiation may be rejected to increase the selectivity andthe signal to noise ratio by rejecting the low energy noise.

The collimators of this invention are preferably fabricated frommaterials that provide high attenuation of the radiation being used tomonitor local power levels. For both fast neutrons and gamma rayradiation, collimators of lead, boron steel, mild steel, and the likemay be used. For fast neutrons, the hydrogeneous plastics may be used,although they will not attenuate gamma rays appreciably. The preferredmaterial is mild steel.

This application is a continuation of my copending application SerialNo. 793,117, filed February 13, 1959, and assigned to a common assignee,now abandoned.

A particular embodiment of this invention has been described inconsiderable detail by way of illustration. It should be understood thatvarious other modifications and adaptations thereof may be made by thoseskilled in this i2 particular .art without departing from. the spiritand scope of this invention as set forth'in the following claims.

I claim:

1. A method for monitoring the power distribution throughout a nuclearchain reacting assembly disposed in a reactor vessel and having aplurality of nuclear fuel-containing coolant flow channels through whicha liquid coolant is passed, which method comprises the steps ofcollimating penetrating radiation emitted axially from a relativelysmall local region of said assembly including at least one of said flowchannels, measuring the intensity of the collimated radiation at a pointremote from said assembly and outside of said vessel, the attenuation ofsaid radiation varying with changes in the density and the temperatureof said coolant through which it passes and the power level in saidchannel from which it issues whereby the collimated radiation has anintensity which is a direct function of said power level, and repeatingthe radiation collimation and collimated radiation intensity measurementsteps for a plurality of said relatively small local regions todetermine said power distribution.

2. A method according to claim 1 in combination with the step ofcontrolling the nuclear chain reacting assembly to maintain apredetermined power distribution therein.

3. A method according to claim 1 wherein said penetrating radiation isemitted axially from an end of at least one coolant flow channel.

4. A method according to claim 1 where said penetrating radiation isemitted laterally from said assembly.

5. In the operation of a nuclear chain reacting as sembly disposed in areactor vessel and having a plurality of nuclear fuel-containing coolantflow channels through which a coolant is passed to absorb heat liberatedby nuclear reaction in said fuel and having a plurality of nuclearreaction poison-containing control elements distributed in said assemblyto control the reaction, the improvement which comprises collimatingpenetrating radiation emitted from said assembly to isolate that emittedfrom each of a plurality of relatively small local regions and toattenuate in each case such radiation emitted from other regions of saidassembly, measuring at a plurality of points remote from said assemblyand outside said vessel the intensity of the collimated radiation fromeach of said plurality of relatively small local regions of saidassembly, and controlling the assembly by means of said local controlelements in accordance with radiation intensities thus measured tomaintain a predetermined power distribution throughout said assembly.

6. An apparatus for monitoring the power distribution in a nuclear chainreacting assembly which comprises means for collimating penetratingradiation emitted from a plurality of relatively small local regions ofsaid assembly, said collimating means comprising an elongated elementfabricated of a material providing a high attenuation of saidpenetrating radiation and having a central opening of relatively smalldiameter, and means at a point remote from said assembly for detectingthe collimated radiation emitted from said plurality of said localregions, said elongated element having a Wall thickness which isrelatively thick at its end adjacent the detecting means and which wallthickness decreases with distance from the detecting means toward thechain reacting assembly.

7. An apparatus for monitoring the power distribution in a nuclear chainreacting assembly which comprises means for collimating penetratingradiation emitted from a plurality of relatively small local regions ofsaid assembly, said collimating means comprising an elongated elementfabricated of a material providing a high attenuation of saidpenetrating radiation and having a central opening of relatively smalldiameter, and means at a point remote from said assembly for detectingthe collimatcd radiation emitted from said plurality of said localregions,

said elongated element having a wall thickness which is relatively greatat its end adjacent the detecting means and which wall thicknessdecreases with distance from the detecting means toward the chainreacting assembly, the shape of said elongated element thusapproximating a hyperboloid of revolution.

8. An apparatus for monitoring the power level in each of a plurality ofrelatively small local regions of a nuclear chain reacting assemblyenclosed within a vessel which comprises a plurality of penetratingradiation detecting means disposed at points remote from said assemblyand outside said vessel, a plurality of radiation collimating meansdisposed between said detecting means and said assembly, each of saidcollimating means being aligned with a corresponding one of saidplurality of relatively small local regions of said assembly, each ofsaid collimating means being disposed immediately adjacent acorresponding one of said detecting means.

9. An apparatus for monitoring the power level in a particular region ofa nuclear chain reacting assembly enclosed within a vessel whichcomprises penetrating radiation detection means disposed at a pointremote from said assembly and outside of said vessel, and radiationcollimating means disposed between said detecting means and saidassembly and aligned with the particular region to be monitored, saidcollimating means including an elongated external collimating elementhaving a central bore and fabricated of material providing highattenuation of said penetrating radiation and disposed immediatelyadjacent said detecting means outside of said vessel,

and an elongated internal collimating element having a central bore anddisposed inside said vessel adjacent said assembly and aligned with thecentral bore of said external collimating element.

10. An apparatus according to claim 9 wherein said internal collimatingelement extends from a point adja cent said nuclear chain reactingassembly to a point adjacent the inner surface of said vessel, saidelement being open at the end adjacent said assembly and closed at theother end and provided with a transversed divider intermediate theseends and with at least one opening in the wall thereof adjacent saiddivider nearest said assembly.

11. An apparatus according to claim 9 wherein said external collimatingelement has a shape which generally tapers toward the radiation source.

References Cited in the file of this patent Proceedings of 1st GenevaConference on Peaceful Uses of Atomic Energy, 1955, vol. 3, pages 214,223, 224, 25 9, 260; published by UN.

Proceedings of 1st Geneva Conference on Peaceful Uses of Atomic Energy,1955, vol. 2, pages 93, 94; pub lished by U.N.

Review of Scientific Instruments, vol. 29, No. 11, 250/ NEU/D, November1958, pages 982, 983.

Abson: Proceedings of 2nd Geneva Conference on Peaceful Uses of AtomicEnergy, 1958, vol. 11, pp. 505- 508; published by UN.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,165,446 January 12 1965 Samuel Untermyer II It is hereby certified thaterror appears in the above numbered patent req'iiring correction andthat the said Letters Patent should read as corrected below.

Column 4, line 25, for "interduced" read introduced --E; column 6, line50, for "With the higher" read With higher column 7, Table I, secondcolumn, line 1 thereof, for "5 0" read 5.0 column 8, line 46, for"atteunated" read attenuated line 48, for "attentuated" read attenuatedcolumn 11, line 35, for "button" read buttons column 11, line 36, for"microns" read micron column 12, line 30 for "where" read wherein Signedand sealed this 15th day of June 1965.

(SEAL) Attest:

ERNEST w. SWIDER EDWARD J. BRENNER Aitesting Officer Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,165,446 January 12, 1965 Samuel Untermyer II It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 4, line 25, for "interduced" read introduced column 6, line 50,for "With the higher" read With higher column 7, Table I, second column,line 1 thereof, for "5 0" read 5.0 column 8, line 46, for "atteunated"read attenuated line 48, for "attentuated" read attenuated column 11,line 35, for "button" read buttons column 11, line 36, for "microns"read micron column 12, line 30 for "where" read wherein Signed andsealed this 15th day of June 1965.

(SEAL) Attest:

- ERNEST w. SWIDER EDWARD J. BRENNER Ana-sting Officer Commissioner ofPatents

1. A METHOD FOR MONITORING THE POWER DISTRIBUTION THROUGHOUT A NUCLEARCHAN REACTING ASSEMBLY DISPOSED IN A REACTOR VESSEL AND HAVING APLURALITY OF NUCLEAR FUEL-CONTAINING COOLANT FLOW CHANNELS THROUGH WHICHA LIQUID COLLANT IS PASSED, WHICH METHOD COMPRISES THE STEPS OFCOLLIMATING PENETRATING RADIATION EMITTED AXIALLY FROM A RELATIVELYSMALL LOCAL REGION OF SAID ASSEMBLY INCLUDING AT LEAST ONE OF SAID FLOWCHANNELS, MEASURING THE INTENSITY OF THE COLLIMATED RADIATION AT A POINTREMOTE FROM SAID ASSEMBLY AND OUTSIDE OF SAID VESSEL, THE ATTENUATION OFSAID RADIATION VARYING WITH CHANGES IN THE DENSITY AND THE TEMPERATUREOF SAID COOLANT THROUGH WHICH IT PASSES AND THE POWER LEVEL IN SAIDCHANNEL FROM WHICH IT ISSUES WHEREBY THE COLIMATED RADIATION HAS ANINTENSITY WHICH IS A DIRECT FUNCTION OF SAID POWER LEVEL, AND REPEATINGTHE RADIATION COLLIMATION AND COLLIMATED RADIATION INTENSITY MEASUREMENTSTEPS FOR A PLURALITY OF SAID RELATIVELY SMALL LOCAL REGIONS TODETERMINE SAID POWER DISTRIBUTION.
 8. AN APPARATUS FOR MONITORING THEPOWER LEVEL IN EACH OF A PLURALITY OF RELATIVELY SMALLL LOCAL REGIONS OFA NUCLEAR CHAIN REACTING ASSEMBLY INCLOSED WITHIN A VESSEL WHICHCOMPRISES A PLURALITY OF PENETRATING RADIATION DETECTING MEANS DISPOSEDAT POINTS REMOTE FROM SAID ASSEMBLY AND OUTSIDE SAID VESSEL, A PLURALITYOF RADIATION COLLIMATING MEANS DISPOSED BETWEEN SAID DETECTING MEANS ANDSAID ASSEMBLY, EACH OF SAID COLLIMATING MEANS BEING ALIGNED WITH ACORRESPONDING ONE OF SAID PLURALITY OF RELATIVELY SMALL LOCAL REGIONS OFSAID ASSEMBLY, EACH OF SAID COLLIMATING MEANS BEING DISPOSED IMMEDIATELYADJACENT A CORRESPONDING ONE OF SAID DETECTING MEANS.